The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Reciprocating pumps such as triplex pumps and quintuplex pumps are generally used to pump high pressure fracturing fluids downhole. An example of a triplex or quintuplex pump is disclosed in commonly assigned PCT Application No. PCT/IB2010/053868, the entire contents of which are hereby incorporated by reference into the current disclosure. Typically, the pumps that are used for this purpose have plunger sizes varying from about 7 cm (2.75 in.) to about 16.5 cm (6.5 in.) in diameter and may operate at pressures up to 144.8 MPa (21,000 psi). In one case, the outer diameter of the plunger is about 9.5 cm (3.75 in) and the reciprocating pump is a triplex pump.
These pumps typically have two sections: (a) a power end, the motor assembly that drives the pump plungers (the driveline and transmission are parts of the power end); and (b) a fluid end, the pump container that holds and discharges pressurized fluid.
In triplex pumps, the fluid end has three fluid cylinders. For the purpose of this document, the middle of these three cylinders is referred to as the central cylinder, and the remaining two cylinders are referred to as side cylinders. A fluid end may comprise a single block having cylinders bored therein, known in the art as a monoblock fluid end. Similarly, a quintuplex pump has five fluid cylinders, including a middle cylinder and four side cylinders.
The pumping cycle of the fluid end is composed of two stages: (a) a suction cycle: During this part of the cycle a piston moves outward in a packing bore, thereby lowering the fluid pressure in the fluid end. As the fluid pressure becomes lower than the pressure of the fluid in a suction pipe (typically 2-3 times the atmospheric pressure, approximately 0.28 MPa (40 psi)), the suction valve opens and the fluid end is filled with pumping fluid; and (b) a discharge cycle: During this cycle, the plunger moves forward in the packing bore, thereby progressively increasing the fluid pressure in the pump and closing the suction valve. At a fluid pressure slightly higher than the line pressure (which can range from as low as 13.8 MPa (2,000 psi) to as high as 144.8 MPa (21,000 psi) the discharge valve opens, and the high pressure fluid flows through the discharge pipe. In some cases, the pump is operated at 12,000 psi. In some other cases, the pump is operated at 15,000 psi. In some further cases, the pump is operated at 20,000 psi.
In assembling and operating these pumps at such harsh conditions, zones of weaknesses have been identified which present a high potential for failure and injury. One example zone includes the tie rods used to connect the power and the fluid end of the pumps, and more particularly the threads at each end of the tie rods. As a precaution, tie rods that are generally used in the industry need to be periodically inspected and replaced in an effort to prevent the tie rod from becoming defective.
It is therefore desirable to provide an improved tie rod and pump equipment to increase efficiency, flexibility, reliability, and maintainability.